1. Field of the Invention
The present invention relates to storage devices and writing methods of the storage devices.
2. Description of the Related Art
Examples of storage devices used for semiconductor integrated circuits and the like include dynamic random access memories (DRAMs) which are volatile storage devices. DRAMs are storage devices storing data by holding electric charge in capacitors.
These days, storage devices including oxide semiconductor (OS) transistors are studied, and nonvolatile storage devices having structures that are similar to the structures of DRAMs are disclosed (see Patent Document 1). An oxide semiconductor has a wide bandgap (for example, approximately 3 eV) and large hole effective mass. Thus, the oxide semiconductor has extremely low off-state leakage current (leakage current of a transistor in an off state). Accordingly, the use of an oxide semiconductor for a transistor in a DRAM can achieve long-term holding of electric charge in a capacitor.
In order to correctly write data to a storage device such as a DRAM, a method for stepping up write voltage from power supply voltage is employed (for example, see Patent Document 2). Further, by stepping up write voltage, on-state current of a write transistor is increased; thus, write time can be shortened.